Aberration correcting optical unit, optical pickup apparatus, and information recording/reproducing apparatus

ABSTRACT

An aberration correcting optical unit includes an optical element for causing a phase change to light passing therethrough by the application of voltage and electrode layers for applying voltages to the optical element. The optical element is sandwiched between the electrode layers. At least one of the electrode layers includes a plurality of electrodes which are electrically isolated from one another. The plurality of electrodes are disposed such that an electric field generated in a portion of the optical element corresponding to a portion between the plurality of electrodes is larger than a predetermined intensity when a predetermined voltage is applied to the optical element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aberration correcting optical unitfor correcting aberration when information is recorded on or reproducedfrom an information recording medium such as an optical disc as well asan optical pickup apparatus having the aberration correcting opticalunit and an information recording and/or reproducing apparatus(hereinafter, referred to as “an information recording/reproducingapparatus”).

2. Description of the Related Art

Optical discs such as a CD (Compact Disc) and a DVD (Digital Video Discor Digital Versatile Disc) are known as information recording media foroptical information recording or reproduction. In addition, a variety ofdifferent optical discs are now under development, such as an opticaldisc specialized for reproduction, a write-once optical disc capable ofadditionally recording information thereon, a rewritable optical disccapable of erasing information therefrom and re-recording informationthereon, and so on.

Research and development is being pursued relating to high recordingdensity optical discs and an optical pickup apparatus for thehigh-density discs. In addition, research and development are now underprogress for a compatible pickup apparatus and informationrecording/reproducing apparatus which are applicable to different typesof optical discs.

It is contemplated that the numerical aperture (NA) of an objective lensprovided in an optical pickup apparatus is increased to irradiate anoptical disc with a light beam of a smaller irradiation diameter forsupporting the higher density trend of the optical disc. It is alsocontemplated that a short wavelength light beam is used to support thehigher density trend.

An increased numerical aperture of an objective lens and the use of ashort wavelength light beam, however, result in larger aberration of thelight beam by the optical disc, causing difficulties in improving theaccuracy of information recording and information reproduction.

For example, an incident angle range of the light beam to the opticaldisc becomes larger as the numerical aperture NA of an objective lens isincreased, thereby resulting in a larger distribution width of thebirefringence amount on the optical disc pupil plane, which is an amountdepending on the incident angle. This causes a problem of a largerinfluence of spherical aberration resulting from the birefringence.Also, when a light beam of a short wavelength is used with an increasednumerical aperture NA of an objective lens, influence of coma aberrationcannot be negligible if the optical disc is inclined during recording orreproducing information so as to incline an incident angle (tilt angle)of the light beam with respect to the normal direction of the opticaldisc.

Further, the influence of aberration such as the above-describedspherical aberration and coma aberration differs depending on the typeof a particular optical disc since different types of optical discs suchas CD and DVD have different structures and recording densities, therebymaking it difficult to develop compatible optical pickup apparatus andinformation recording/reproducing apparatus.

Conventionally, an optical pickup apparatus having a liquid crystal unitfor correcting aberration has been proposed for reducing the influenceof the aberration as mentioned above (Laid-open Japanese PatentApplication Kokai No. H10-20263).

This liquid crystal unit has a structure in which a liquid crystalelement C is sandwiched between mutually opposing transparent electrodesA, B, as schematically illustrated in FIG. 1. A voltage applied betweenthe transparent electrodes A, B is adjusted to change the alignmentstate of the liquid crystal element C, such that when light incident onone of the transparent electrode A (or B) passes through the liquidcrystal element C, a change in birefringence is given to the light inaccordance with the alignment state to emit the light to the othertransparent electrode B (or A).

Further, at least one of the transparent electrodes A, B is divided intoa plurality of transparent electrodes, for example, a1, a2, a3 and b1,b2, b3. Also, the transparent electrodes a1, a2, a3 are electricallyisolated from one another, while the transparent electrodes b1, b2, b3are also electrically isolated from one another.

Therefore, the liquid crystal element C can be adjusted in a pluralityof different alignment states when different voltages are appliedbetween transparent electrodes in an opposing relationship, for example,between the transparent electrodes a1, b1; between the transparentelectrodes a2, b2; and between the transparent electrodes a3, b3, sothat changes in birefringence in accordance with the respectivealignment states can be simultaneously given to light incident thereon.

Then, the liquid crystal unit is positioned on an optical path between alight source for emitting laser light and an objective lens. The liquidcrystal unit gives changes in birefringence in accordance with theplurality of alignment states to the laser light, causing the laserlight to transmit therethrough to the objective lens. The objective lensconverges the transmitted laser light to generate a light beam which isirradiated to an optical disc. Also, when reflected light produced byirradiating the optical disc with the light beam impinges on the liquidcrystal unit through the objective lens, the reflected light is giventhe changes in birefringence in accordance with the plurality ofalignment states, causing the reflected light to transmit, and thetransmitted reflected light is detected by a photodetector. Therefore,the plurality of alignment states of the liquid crystal unit areadjusted as appropriate to reduce the influence of aberration such asspherical aberration and coma aberration.

However, gaps (SP) are provided between the respective transparentelectrodes in the conventional liquid crystal unit to electricallyisolate the plurality of transparent electrodes a1, a2, a3 and b1, b2,b3, as illustrated in FIG. 1. More specifically, the gaps SP areprovided along the respective boundaries of the transparent electrodesa1, a2, a3, and the gaps SP are provided along the respective boundariesof the transparent electrodes b1, b2, b3.

Therefore, no voltage is applied to the gaps SP, so that the foregoingstructure suffers from an inability of controlling the alignment statesin the liquid crystal element C corresponding to the gaps SP. As aresult, the aberration correction can be made for a light beam orreflected light passing through the transparent electrodes a1, a2, a3and b1, b2, b3, whereas no aberration correction can be made for a lightbeam or reflected light passing through the gaps SP, so that a highlyaccurate aberration correction cannot be carried out for the light beamor the reflected light.

Also, when the transparent electrodes are divided by a larger numberwith the intention of making a finer correction for the influence ofaberration, a large number of transparent electrodes are formed withrequired electrical insulating features therebetween within a limitedeffective optical path range in which the laser light or reflected lightpasses, resulting in an increased number of the gaps SP and a largerarea occupied thereby. As a result, a fine aberration correction becomesdifficult.

Further, different voltages applied to mutually adjoining transparentelectrodes result in abrupt discontinuous alignment states produced inthe liquid crystal element C corresponding to the gaps SP interveningbetween the transparent electrodes.

OBJECT AND SUMMARY OF THE INVENTION

The present invention has been made to overcome the problems of theprior art as mentioned above. It is, therefore, an object of the presentinvention to provide an aberration correcting optical unit capable ofaccurately correcting the influence of aberration due to an informationrecording medium, as well as an optical pickup apparatus including theaberration correcting optical unit, and an informationrecording/reproducing apparatus including the optical pickup apparatus.

It is another object of the present invention to provide an aberrationcorrecting optical unit capable of accurately correcting the influenceof aberration associated with the higher density trend of theinformation recording media, as well as an optical pickup apparatusincluding the aberration correcting optical unit, and an informationrecording/reproducing apparatus including the pickup apparatus.

According to the present invention, there is provided an aberrationcorrecting optical unit disposed in an optical path of an optical systemfor irradiating a recording medium with a light beam emitted from alight source and for guiding a reflected light beam reflected by therecording medium, for correcting aberration occurring in the opticalpath, which comprises an optical element for causing a phase change tolight passing therethrough by the application of voltage; and electrodelayers for applying voltages to the optical element, the electrodelayers sandwiching the optical element, wherein at least one of theelectrode layers includes a plurality of electrodes electricallyisolated from one another, and the plurality of electrodes are disposedsuch that an electric field generated in a portion of the opticalelement corresponding to a portion between the plurality of electrodesis larger than a predetermined intensity when a predetermined voltage isapplied to the optical element.

According to the present invention, there is provided an aberrationcorrecting optical unit disposed in an optical path between a lightsource and an optical element for irradiating an information recordingmedium with a light beam emitted from the light source, in alignmentwith an optical axis, for correcting aberration of light caused by theinformation recording medium, which comprises a liquid crystal elementexhibiting a predetermined alignment state by applying a predeterminedvoltage; and mutually opposing electrodes for applying voltages to theliquid crystal element, wherein at least one of the mutually opposingelectrodes is formed with a plurality of electrodes of a multi-layerstructure arranged in the direction of the optical axis.

According to another aspect of the present invention, the plurality ofelectrodes formed in a multi-layer structure is formed such that theplurality of electrodes are arranged in the direction of the opticalaxis without overlapping one another.

According to another aspect of the present invention, the plurality ofelectrodes formed in a multi-layer structure is formed such that theplurality of electrodes partially overlap one another, and are arrangedin the direction of the optical axis.

According to another aspect of the present invention, the electrodes areapplied with voltages which produce an electro-optical effect oppositeto aberration characteristics caused by an information recording medium.

According to the aberration correcting optical unit of the presentinvention having the foregoing structure, an electro-optical effect ofcharacteristics opposite to aberration characteristics caused by theinformation recording medium can be produced in the liquid crystalelement by adjusting the voltages applied across mutually opposingvoltages, and aberration of light transmitting the liquid crystalelement can be corrected by this electro-optical effect. Further, whenat least one of the mutually opposing electrodes is formed of aplurality of electrodes in a multi-layer structure, the plurality ofelectrodes can be oriented toward the liquid crystal element withoutgaps. For this reason, as the plurality of electrodes are applied withvoltages, the electro-optical effect can be produced in the liquidcrystal element without gaps, thereby making it possible to correct theaberration without omission. It is also possible to finely correct theaberration.

According to the present invention, there is provided an aberrationcorrecting optical unit disposed in an optical path between a lightsource and an optical element for irradiating an information recordingmedium with a light beam emitted from the light source, in alignmentwith an optical axis, for correcting aberration of light caused by theinformation recording medium, which comprises a liquid crystal elementexhibiting a predetermined alignment state by applying a predeterminedvoltage; and mutually opposing electrodes for applying voltages to theliquid crystal element, wherein at least one of the mutually opposingelectrodes is formed with a plurality of electrodes of a multi-layerstructure arranged in the direction of the optical axis.

According to the configuration as described, information can beaccurately reproduced based on the reflected light which has beencorrected for the influence of aberration.

An information reproducing apparatus according to the present inventioncomprises the above-mentioned pickup apparatus, and reproducesinformation by emitting information recording light, and detectingreflected light from an information recording medium. According to thisconfiguration, information can be accurately reproduced based on thereflected light which has been corrected for the influence ofaberration.

An information recording apparatus according to the present inventioncomprises the above-mentioned pickup apparatus, and records informationon an information recording medium by emitting information recordinglight. According to the configuration, information can be accuratelyreproduced based on reflected light reflected from the informationrecording medium and corrected for the influence of aberration.

According to the present invention, there is provided an aberrationcorrecting optical unit disposed in an optical path of an optical systemfor irradiating a recording medium with a light beam emitted from alight source and for guiding a reflected light beam reflected by therecording medium, for correcting aberration occurring in the opticalpath, which comprises a liquid crystal element for causing a phasechange to light passing therethrough by the application of voltage;mutually opposing electrode layers for applying the liquid crystalelement with voltages; and an insulating layer disposed between theliquid crystal element and at least one of the electrode layers, whereinthe at least one of the electrode layer includes a plurality of dividedelectrodes electrically isolated from one another by gaps in the sameplane, and the insulating layer is of a thickness such that an electricfield generated in a portion of the optical element corresponding to thegap between the plurality of divided electrodes is larger than apredetermined intensity when a predetermined voltage is applied to theoptical element.

According to the present invention, there is provided an optical pickupapparatus including the aberration correcting optical unit mentionedabove, which comprises a light source for emitting a light beam; anoptical system for irradiating a recording medium with the light beamemitted from the light source and for guiding a reflected light beamreflected by the recording medium; and a photodetector for detecting thereflected light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating a conventional liquidcrystal unit;

FIG. 2 is a diagram illustrating the configuration of an optical pickupapparatus provided in an information recording/reproducing apparatusaccording to an embodiment of the present invention;

FIG. 3 is a diagram illustrating the operational principle of anaberration correcting optical unit provided in the optical pickupapparatus;

FIG. 4 is a top plan view illustrating the shape of the aberrationcorrecting optical unit;

FIG. 5 is a cross-sectional view illustrating the verticalcross-sectional structure of the aberration correcting optical unit;

FIG. 6 is a diagram illustrating the principle of the aberrationcorrection by means of the aberration correcting optical unit;

FIG. 7 is a top plan view illustrating the shape of another aberrationcorrecting optical unit;

FIG. 8 is a cross-sectional view illustrating the verticalcross-sectional structure of the aberration correcting optical unit inFIG. 7;

FIG. 9 is a graph showing the characteristic of coma aberration;

FIG. 10 is a characteristic graph showing the result of reducing thecoma aberration by means of the aberration correcting optical unit;

FIG. 11 is a graph showing the characteristic when the coma aberrationis not sufficiently reduced;

FIG. 12 is a cross-sectional view illustrating another structure of theaberration correcting optical unit;

FIG. 13 is a cross-sectional view illustrating a further anotherstructure of the aberration correcting optical unit;

FIG. 14 is a cross-sectional view illustrating a further anotherstructure of the aberration correcting optical unit;

FIG. 15 is a perspective view schematically illustrating the structureof an aberration correcting liquid crystal unit and a change inalignment of crystal molecules;

FIG. 16 is a top plan view of an aberration correcting optical unit forcorrecting spherical aberration;

FIG. 17 is a top plan view of an aberration correcting optical unit forcorrecting coma aberration;

FIG. 18 is a cross-sectional view illustrating the structure of theaberration correcting optical unit illustrated in FIG. 16, taken along aline X-X;

FIG. 19 is a cross-sectional view for explaining electric fields over aliquid crystal element in aberration correcting regions and gap regionswhen each transparent electrode is applied with a voltage;

FIG. 20 shows graphs of the electric field intensities on interfacesbetween respective insulating layers and liquid crystal element, withthe thickness (THi) of the insulating layers used as a parameter, wheneach electrode is applied with a voltage;

FIG. 21 is a graph showing an example of wave front aberration(spherical aberration) produced in a light beam by an optical disc;

FIG. 22 is a graph showing a phase difference (nm) produced in a lightbeam passing through the liquid crystal element when predeterminedvoltages are applied to the liquid crystal element, when the thicknessof an insulating layer is smaller than the thickness of the liquidcrystal element;

FIG. 23 is a graph showing a phase difference (nm) produced in a lightbeam passing through the liquid crystal element when predeterminedvoltages are applied to the liquid crystal element when the thickness ofthe insulating layer is equal to or larger than the thickness of theliquid crystal element; and

FIG. 24 is a graph showing a phase difference (nm) produced in a lightbeam passing through the liquid crystal element when the thickness ofthe insulating layer is smaller than the thickness of the liquid crystalelement in another case.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will hereinafter be described withreference to the accompanying drawings. FIG. 2 is a diagram illustratingthe configuration of an optical pickup apparatus provided in aninformation recording/reproducing apparatus.

In FIG. 2, the optical pickup apparatus PU includes a light source 1 foremitting laser light H1, a polarizing beam splitter 3, an aberrationcorrecting optical unit 4, an objective lens 5, a converging lens 6 anda photodetector 7. The components 1-7 are arranged along an optical axisOA. A control circuit 8 is provided in the optical pickup apparatus PUor in an information recording/reproducing apparatus for controlling theaberration correcting optical unit 4.

The aberration correcting optical unit 4 has an electro-optic elementwhich exhibits an electro-optic effect that varies depending on anelectric field applied thereto. More specifically, the aberrationcorrecting optical unit 4 has a liquid crystal optical element.Birefringence characteristics of the liquid crystal optical elementchanges in response to a control voltage Vi applied thereto by thecontrol circuit 8.

In particular, the aberration correcting optical unit 4 has a structure,as schematically illustrated in FIG. 3, in which a liquid crystalelement 14 is encapsulated between two transparent insulating substrates10, 11 such as glass substrates. Formed between the opposing surfaces ofthe glass substrates 10, 11 are electrodes 12, 13; insulating films 23,24; and liquid crystal alignment films 21, 22.

The alignment state of the liquid crystal element 14 changes when acontrol voltage Vi is applied between the electrodes 12, 13, in responseto an electric field Ei produced by the control voltage Vi. As a result,light passing through the liquid crystal element 14 receivesbirefringence of the liquid crystal element 14 to change in a polarizingstate (phase). The polarizing state (phase) can be controlled by thecontrol voltage Vi applied to the liquid crystal element 14.

The aberration correcting optical unit 4 also has bidirectional lighttransmissivity, so that any side of the insulating substrates 10, 11 maybe oriented to the objective lens 5.

The aberration correcting optical unit 4 is partitioned into a pluralityof aberration correcting regions AR1-ARi which have been determined incorrespondence to the distribution of aberration caused by the opticaldisc 9, as illustrated in a top plan view of FIG. 4. The aberrationcorrecting regions AR1-ARi are implemented by transparent electrode(ITO: Indium Tin Oxide) layers formed in the electrodes 12, 13. Itshould be noted that while FIG. 4 illustrates a typical example of theaberration correcting regions AR1, AR2-ARi for correcting sphericalaberration caused by the optical disc 9, the aberration correctingoptical unit 4 is actually partitioned into a variety of shapes incorrespondence to the distribution of particular aberration caused bythe optical disc 9. For example, for correcting coma aberration causedby the optical disc 9 which is inclined during recording or reproductionof information, the aberration correcting optical unit 4 is providedwith aberration correcting regions BR1-BR9 of shapes as illustrated inFIG. 7. Also, the number of sections of the aberration correctingregions is determined in correspondence to the distribution ofaberration caused by the optical disc 9.

When the aberration correcting optical unit 4 is provided with theconcentric aberration correcting regions AR1-ARi as illustrated in FIG.4, the electrode 12 includes transparent electrode layers A1-Ai embeddedin a transparent insulating layer 15 in an electrically isolatedrelationship with each other; and transparent electrode layers B1-Bjembedded in the insulating layer 15 opposed to a plurality of gaps W1existing between the respective transparent electrode layers A1-Ai, asillustrated in a cross-sectional view of FIG. 5 (a diagram illustratingthe cross-sectional structure taken along a line X-X in FIG. 4). Also,the group of the transparent electrodes A1-Ai and the group of thetransparent electrode layers B1-Bj are formed in a two-stage structurewithin the insulating layer 15 along the optical axis OA. Forconvenience of description, the insulating films 23, 24 and the liquidcrystal alignment films 21, 22 are omitted in the cross-sectional viewof FIG. 5 (the same is applied likewise to FIGS. 6, 8, 12, 13, 14).

The transparent electrode layer A1 is formed in a shape that conforms tothe aberration correcting region AR1 (circular in FIG. 4); thetransparent electrode A2 is formed in a shape that conforms to to theaberration correcting region AR2 (annular in FIG. 4); and the remainingtransparent electrode layers A3-Ai are likewise formed in shapes whichconform to the corresponding aberration correcting regions AR3-ARi.

The transparent electrode layer B1 in turn is formed in an extremelynarrow annular shape that conforms to the shape of the gap W1 forelectrically isolating the transparent electrode layers A1, A2. Thetransparent electrode layer B2 is likewise formed in an extremely narrowannular shape that conforms to the shape of the gap W1 for electricallyisolating the transparent electrode layers A2, A3, and the remainingtransparent electrode layers B3-Bj are formed in a similar manner.

In other words, the aberration correcting regions AR1-ARi illustrated inFIG. 4 are implemented by forming the transparent electrodes A1-Ai in anelectrically isolated structure, while the respective gaps BK1-BKjbetween the aberration correcting regions AR1-ARi are implemented byforming the transparent electrodes B1-Bj.

It should be noted that while these transparent electrodes B1-Bj may bearranged opposed to all of the existing gaps W1, they may be formedopposed to a number of gaps W1 which may be required in accordance withthe characteristic of a particular aberration to be corrected.

On the other hand, the electrode 13 has a similar two-stage structurecomprised of a group of transparent electrode layers C1-Ci embedded in atransparent insulating layer 16 in an electrically isolated relationshipwith each other, and a group of transparent electrode layers D1-Djembedded in the insulating layer 16 opposed to a plurality of gaps W2existing between the respective transparent electrodes C1-Ci. Thetransparent electrode layers C1-Ci are in an opposed relationship withthe transparent electrode layers A1-Ai of the electrode 12, while thetransparent electrode layers D1-Dj are in an opposing relationship withthe transparent electrode layers B1-Bj of the electrode 12.

It should be noted that while the transparent electrode layers D1-Dj mayalso be arranged opposed to all of the existing gaps W2, they may beformed opposed to a number of gaps required in accordance with thecharacteristic of particular aberration to be corrected.

Then, as schematically illustrated in FIG. 6, when appropriate,different voltages V1-Vk are applied respectively across the respectivetransparent electrode layers (A1, C1)-(Ai, Ci) of the aberrationcorrecting regions AR1-ARi in a mutually opposed relationship as well asthe respective transparent electrode layers (B1,D1)-(Bj, Di) of the gapsBK1-BKj by the control voltage Vi from the control circuit 8, theapplied voltages V1-Vk cause a plurality of alignment states to occurwithin the liquid crystal element 14. The voltages V1-Vk are determinedto be such voltages that provide the alignment states of the liquidcrystal element 14 in the respective aberration correcting regionsAR1-ARi and the gaps BK1-BKj with the characteristics opposite to thecharacteristic of aberration caused by the optical disc 9.

In this way, even with the gaps BK1-BKj interposed between therespective aberration correcting regions AR1-ARi, the transparentelectrode layers B1-Bj, D1-Dj are provided for the gaps BK1-Bkj, so thatthe alignment states can be adjusted for correcting aberration over theentire region of the liquid crystal element 14, and the alignment statescan be finely adjusted.

Also, as the voltages V1-Vk across the respective transparent electrodelayers (B1, D1)-(Bj, Di) are adjusted as appropriate, their alignmentstates can be changed continuous with the alignment states produced bythe voltages applied across the transparent electrode layers (A1,C1)-(Ai, Ci), without causing any abrupt change.

Likewise, with an aberration correcting optical unit 4 having theaberration correcting regions BR1-BR9 for correcting coma aberration asillustrated in FIG. 7, transparent electrode layers (reference numeralsof which are omitted) corresponding to the aberration correcting regionsBR1-BR9, and transparent electrode layers corresponding to gaps betweenthe aberration correcting regions BR1-BR9 are formed on the electrodes12, 13, respectively, in a two-stage structure, as illustrated in across-sectional view of FIG. 8 (which is a diagram illustrating thecross-sectional structure taken along a line X-X in FIG. 7). It shouldbe noted that FIG. 8 shows that the transparent electrode layerscorresponding to the gaps between the respective aberration correctingregions BR1-BR9 are not formed opposed to all the gaps but are providedonly for those gaps in which the influence of coma aberration should beparticularly reduced, as indicated by reference numerals F1-F4, G1-G4.

The operation of the optical pickup apparatus PU having the aberrationcorrecting optical unit 4 constructed as described will be explainedwith reference to FIGS. 2 and 7-11. The operation of the apparatus willbe explained as a representative example in which the optical pickupapparatus PU is provided with the aberration correcting optical unit 4for correcting coma aberration illustrated in FIGS. 7 and 8.

When an optical disc 9 is loaded at a so-called clamp position in aninformation recording/reproducing apparatus, and the user instructs theinformation recording/reproducing apparatus to start reproduction ofinformation, a system controller (not shown) provided in the informationrecording/reproducing apparatus commands the control circuit 8 to outputa control voltage Vi for correcting coma aberration. Consequently,appropriate voltages are applied respectively across the transparentelectrode layers in the opposed relationship corresponding to therespective aberration correcting regions BR1-BR9 of the aberrationcorrecting optical unit 4 illustrated in FIGS. 7 and 8, and across thetransparent electrode layers F1-F4, G1-G4 in the opposed relationshipcorresponding to the gaps, causing a plurality of alignment states tochange in accordance with associated electric fields produced by theapplied voltages in the liquid crystal element 14.

The system controller drives a spindle motor (not shown) provided in theinformation recording/reproducing apparatus for rotation. The systemcontroller also drives a carriage (not shown) for moving the opticalpickup apparatus PU in a radial direction of the optical disc 9. Theoptical disc 9 is rotated at a predetermined line velocity.

Further, the light source 1 emits linearly polarized laser light H1having a constant power as the system controller supplies a drivingsignal to the light source 1. The laser beam H1 is transformed intocollimated beam by a collimator lens 2. The collimated beam, then,transmits the polarizing beam splitter 3, and is incident on theaberration correcting optical unit 4.

The laser beam incident on the aberration correcting optical unit 4 issubjected to birefringence in accordance with the alignment state of theliquid crystal element 14 when it transmits the aberration correctingoptical unit 4. The laser beam subjected to the birefringence isconverged by the objective lens 5, and a resulting light beam having asmaller irradiation diameter is irradiated on the optical disc 9.

Further, reflected light produced by the light beam reflected off apupil plane of the optical disc 9 is incident on the objective lens 5.The reflected light transmitting the objective lens 5 is again subjectedto the birefringence by the aberration correcting optical unit 4 andtransmits the same, and then is reflected by the polarizing beamsplitter 3 toward the converging lens 6. Then, the reflected light isconverged by the converging lens 6 to be received by the photodetector7. The photodetector 7 transduces the received reflected light to outputan optoelectrically-transduced signal having information recorded on theoptical disc 9. The transduced signal is supplied to a reproduced signalprocessing circuit (not shown) provided in the informationrecording/reproducing apparatus. Finally, the reproduced signalprocessing circuit performs decoding and so on based on the transducedsignal to produce a reproduced signal such as an audio signal and/or avideo signal.

Coma aberration occurs on the pupil plane of the optical disc 9 when theoptical disc 9 is inclined and an incident angle of the light beam isinclined (tilt angle) with respect to the normal direction of theoptical disc 9.

FIG. 9 is a graph which shows the influence of the coma aberrationoccurring on the pupil plane of the optical disc 9. The curve representsa normalized wave front aberration amount, where the horizontal axisrepresents an effective optical path range of the objective lens 5(i.e., lens diameter).

When the coma aberration as shown in FIG. 9 occurs, the aberrationcorrecting optical unit 4 causes birefringence to the laser beamincident thereon from the polarizing beam splitter 3 so as to reduce theinfluence of the coma aberration as described above, so that the opticaldisc 9 is irradiated beforehand with a light beam which can reduce theinfluence of the coma aberration through the objective lens 5. Further,when reflected light returning from the optical disc 9, influenced bythe coma aberration, is again incident on the aberration correctingoptical unit 4 through the objective lens 5, the light beam is given thebirefringence to reduce the influence of the coma aberration on thereflected light, and passes through the aberration correcting opticalunit 4 to the polarizing beam splitter 3. Consequently, the reflectedlight with suppressed influence of the coma aberration is incident onthe photodetector 7 through the converging lens 6, thereby enablinghighly accurate information reproduction.

Further, the gaps between the respective aberration correcting regionsBR1-BR9 of the aberration correcting optical unit 4 are provided withthe transparent electrode layers F1-F4, G1-G4 in an opposingrelationship as illustrated in FIGS. 7 and 8, so that the comaaberration can be reduced over the entire aberration correcting opticalunit 4, and the coma aberration can be accurately reduced.

FIG. 10 is a graph showing the influence of coma aberration reduced bythe aberration correcting optical unit 4, which is represented as anormalized wave front aberration amount, wherein the horizontal axisrepresents an effective optical path range of the objective lens 5(i.e., lens diameter). As is apparent from FIG. 10, a significantimprovement is recognized as compared with the coma aberration beforethe correction shown in FIG. 9.

FIG. 11 in turn shows the influence of coma aberration when only theaberration correcting regions BR1-BR9 are used to correct the aberrationwithout providing the transparent electrode layers in the gaps betweenthe respective aberration correcting regions BR1-BR9 of the aberrationcorrecting optical unit 4, i.e., when the aberration is correctedwithout providing the transparent electrode layers F1-F4, G1-G4illustrated in FIG. 8, for the purpose of comparison with thecharacteristic graph of FIG. 10.

It can be seen that the graph of FIG. 11 presents large peaks P1-P4 inthe aberration in portions where the transparent electrode layers F1-F4,G1-G4 are not provided, whereas the peaks are largely reduced as shownin FIG. 10.

As is also apparent from the result of the experiment described above,it is confirmed that according to the aberration correcting optical unit4 of the embodiment, the influence of the coma aberration can be largelyreduced by the transparent electrode layers F1-F4, G1-G4 providedbetween the gaps between the respective aberration correcting regionsBR1-BR9.

The operation of the optical pickup apparatus PU for recordinginformation will be described.

As the user instructs the information recording/reproducing apparatus tostart recording of information, the system controller provided in theinformation recording/reproducing apparatus commands a recording signalprocessing circuit (not shown) to perform modulation, encoding and so onbased on an input signal such as an audio signal and a video signalsupplied from the outside, and provides the light source 1 with arecording signal produced by such processing, causing the light source 1to emit laser light H1 modulated by the recording signal.

The laser beam H1 is transformed into collimated light beam by thecollimator lens 2, transmits the polarizing beam splitter 3, and isincident on the aberration correcting optical unit 4. The laser beamincident on the aberration correcting optical unit 4 is subjected tobirefringence in accordance with the alignment state of the liquidcrystal element 14 when it transmits the aberration correcting opticalunit 4. The laser beam subjected to the birefringence is converged bythe objective lens 5, and a resulting light beam having a smallerirradiation diameter is irradiated to the optical disc 9 for recordinginformation with optical energy of the light beam.

Further, reflected light produced by the light beam reflected from apupil plane of the optical disc 9 is incident on the objective lens 5.The reflected light transmitting the objective lens 5 is again subjectedto the birefringence by the aberration correcting optical unit 4 andtransmits the same, and then is reflected by the polarizing beamsplitter 3 toward the converging lens 6. Then, the reflected light isconverged by the converging lens 6, such that the resulting convergedlight beam is received by the photodetector 7. The photodetector 7optoelectrically transduces the received reflected light to output thetransduced signal, which is supplied to a servo circuit (not shown)provided in the information recording/reproducing apparatus.

The servo circuit detects a focus error, for example, by an astigmatismmethod, and drives the objective lens 5 in a focus servo scheme based onthe result of the detection. Since the focus servo is performed based onthe optoelectrically transduced signal with significantly reducedinfluence of coma aberration, highly accurate focus servo can beaccomplished.

When the optical pickup apparatus is provided with the aberrationcorrecting optical unit 4 for correcting spherical aberrationillustrated in FIGS. 4 and 5, the pickup apparatus can have similareffects to those produced when it is provided with the aberrationcorrecting optical element 4 for correcting coma aberration illustratedin FIGS. 7 and 8, so that the influence of various types of aberrationcan be significantly reduced.

As described above, according to the optical pickup apparatus PU and theinformation recording/reproducing apparatus of the embodiment, it ispossible to significantly reduce the influence of aberration by theoptical disc 9 and to finely control the reduction of the aberration,since the optical pickup apparatus PU is provided with the aberrationcorrecting optical unit 4, and the transparent electrode layers aredisposed corresponding to the gaps defined between the respectiveaberration correcting regions of the aberration correcting optical unit4, as illustrated in FIGS. 5 and 8.

While the foregoing embodiment has been described for the aberrationcorrecting optical unit 4 which includes the transparent electrodelayers corresponding to the gaps defined between the respectiveaberration correcting regions, the aberration correcting optical unit ofthe present invention is not limited to such a structure.

Alternatively, as illustrated in a cross-sectional view of FIG. 12,transparent electrode layers may be provided corresponding to gapsdefined between respective aberration correcting regions on oneelectrode 12, while a transparent electrode layer 17 is provided overthe entirety of the effective optical path range on the other electrode13, such that the transparent electrode layer 17 is used as a commonelectrode to apply appropriate voltages respectively between therespective transparent electrode layers in the electrode 13 and thetransparent electrode layer 17.

Also alternatively, as illustrated in a cross-sectional view of FIG. 13,a plurality of transparent electrode layers overlapping one another maybe formed along the optical axis OA in a multi-stage structure in eachof the electrodes 12, 13, such that appropriate voltages may be appliedrespectively between the transparent electrode layers in a mutuallyopposing relationship having shapes corresponding to associatedaberration correcting regions. In other words, portions of therespective transparent electrode layers opened to the liquid crystalelement 14 are made to conform to the shapes of the aberrationcorrecting regions, thereby allowing for a correction of aberration.Since such a structure is free from gaps otherwise existing between theaberration correcting regions, the transparent electrode layers such asthose illustrated in FIGS. 5 and 8 are not required corresponding to thegaps. Further effects can be provided in realizing simplification of thecontrol circuit 8 resulting from a reduction in the number of wiringsfor applying voltages to the transparent electrode layers, and areduction in the number of types of voltages to be applied, and so on.

Also, in the structure of FIG. 13, a single transparent electrode layer,similar to the transparent electrode layer 17 illustrated in FIG. 12,may be formed on one electrode 12 (or 13), such that the singletransparent electrode layer is used as a common electrode.

Further alternatively, as illustrated in a cross-sectional view of FIG.14, transparent electrode layers of shapes corresponding to aberrationcorrecting regions may be formed in a multi-stage structure along theoptical axis OA, such that appropriate voltages are respectively appliedacross transparent electrode layers in a mutually opposing relationship.

According to the foregoing structure, it is possible to finely adjustthe alignment states which should be caused in the liquid crystalelement 14, since the shapes of the transparent electrode layers can beformed to conform to the shapes of the aberration correcting regions.Thus, realization of more accurate aberration correcting optical unitcan be attained. Also, the transparent electrode layers such as thoseillustrated in FIGS. 5 and 8 are not required corresponding to the gaps,since no gaps exist between transparent electrode layers. Furthereffects can be provided in realizing simplification of the controlcircuit 8 resulting from a reduction in the number of wirings forapplying voltages to the transparent electrode layers, and a reductionin the number of types of voltages to be applied, and so on.

As described above, according to the foregoing embodiment, at least oneof mutually opposing electrodes for applying a voltage to the liquidcrystal element of the aberration correcting optical unit is formed by aplurality of electrodes in a multi-layer structure, so that theplurality of electrodes can be oriented toward the liquid crystalelement without gaps. Thus, an electro-optical effect is produced in theliquid crystal element without any gap when voltages are applied to theplurality of electrodes, so that the aberration can be corrected withoutany omission. Also, the aberration can be finely corrected.

Further, it is possible to promote a higher NA of an objective lens anda shorter wavelength for light emitted from a light source, which isassociated with the trend of increasing the density of the informationrecording media, because of the ability of appropriately correctingaberration by an information recording medium, thereby providing anaberration correcting optical unit which is effective for the trend ofincreasing the density.

Also, highly accurate information recording and information reproductioncan be accomplished since the optical pickup apparatus as well as theinformation reproducing apparatus and the information recordingapparatus of the present invention comprise the aforementionedaberration correcting optical element to make an aberration correction.

The following description will be made on a liquid crystal element andan aberration correcting optical unit which has insulating layersbetween electrode layers according to another embodiment of the presentinvention.

In the embodiment, an optical unit 4 has a structure in which a liquidcrystal element 114 is sandwiched between two transparent insulatingsubstrates 110, 111 such as glass substrates, as schematicallyillustrated in FIG. 15. There are formed electrodes 112, 113, insulatinglayers 123, 124, and liquid crystal alignment layers 121, 122,respectively, on the mutually opposed surfaces of the insulatingsubstrates 110, 111.

Alignment of liquid crystal molecules in the liquid crystal element 114changes in response to an electric field Ei produced by a controlvoltage Vi which is applied between the electrodes 112, 113. As aresult, light passing through the liquid crystal element 114 is subjectto birefringence of the liquid crystal element 114 to change the phase.The polarization state (phase) can be controlled by the control voltageVi applied to the liquid crystal element 114.

The aberration correcting optical unit 4 also has bidirectional lighttransmissivity, so that any side of the insulating substrates 10, 11 maybe oriented to the objective lens 5.

The aberration correcting optical unit 4 is partitioned into a pluralityof aberration correcting regions AR1-ARi which have been determined incorrespondence to the distribution of aberration caused by the opticaldisc 9, as illustrated in a top plan view of FIG. 16. The aberrationcorrecting regions AR1-ARi are implemented by transparent electrode(ITO: indium tin oxide) layers formed in the electrodes 112, 113. Itshould be noted that while FIG. 16 illustrates a typical example of theaberration correcting regions AR1-ARi for correcting sphericalaberration caused by the optical disc 9, the aberration correctingoptical unit 4 is actually partitioned into a variety of shapes incorrespondence to the distribution of aberration caused by the opticaldisc 9. For example, the aberration correcting optical unit 4 isprovided with aberration correcting regions BR1-BRi of shapes asillustrated in FIG. 17 for correcting coma aberration caused by theoptical disc 9 which is inclined during recording or reproduction ofinformation. Also, the number of sections of these aberration correctingregions is determined in correspondence to the distribution ofaberration by the optical disc 9.

In the following, description will be made on an example of theaberration correcting optical unit 4 which is provided with concentricaberration correcting regions AR1-ARi as illustrated in FIG. 16. FIG. 18is a cross-sectional view illustrating the cross-sectional structuretaken along a line X-X in FIG. 16. As illustrated, the electrode 112 hasa structure comprised of transparent electrodes A1-Ai electricallyisolated from one another by a plurality of gaps P1-Pi existing betweenthe respective transparent electrodes A1-Ai.

The transparent electrode A1 is formed in a shape that conforms to theaberration correcting region AR1 (circular in FIG. 16); the transparentelectrode A2 is formed in a shape that conforms to the aberrationcorrecting region AR2 (annular in FIG. 16); and the remainingtransparent electrodes A3-Ai are likewise formed in shapes which conformto the corresponding aberration correcting regions AR3-ARi. Also, thegaps P1-Pi isolating the transparent electrodes A1-Ai are formed in anannular shape.

The electrode 113 has a similar structure comprised of a plurality oftransparent electrodes C1-Ci electrically isolated from one another by aplurality of gaps Q1-Qi existing between the respective transparentelectrodes C1-Ci.

It should be noted that the electrode 113 need not be isolated if theelectrode 112 is formed as isolated electrodes. For example, theelectrode 113 may be formed as a single electrode extending over theentire plane of the layer, or may be formed in a shape required inaccordance with the characteristic of particular aberration to becorrected, or formed separately in a required number.

It is disclosed that spherical aberration and coma aberration can becorrected by a single liquid crystal unit in Laid-open Japanese PatentApplication Kokai No. H10-289465. Specifically, in the presentinvention, an upper electrode may be formed in a shape for correctingspherical aberration, while a lower electrode may be formed in a shapefor correcting coma aberration, to correct the spherical aberration andthe coma aberration with a single liquid crystal unit. In this event, aninsulating layer between the upper electrode and a liquid crystalelement and an insulating layer between the lower electrode and theliquid crystal element may have suitable thicknesses for allowing thecorrections of the associated aberration, respectively.

Referring to FIG. 19, the following description will be made on thecorrection of aberration when the respective transparent electrodes areapplied with voltages. For convenience of description, FIG. 19illustrates a cross-section of the liquid crystal element in the radialdirection from the center of the element. As schematically illustratedin FIG. 19, when different predetermined voltages V1-Vk are applied bythe control circuit 8 across the respective transparent electrodes (A1,C1)-(Ak, Ck) of aberration correcting regions AR1-ARk which are in amutually opposed relationship, a plurality of alignment states occur inthe liquid crystal element 114 in accordance with electric fields(E1-Ek) produced by the applied voltages V1-Vk. The applied voltagesV1-Vk are determined such that the alignment states of the liquidcrystal element 114 in the respective aberration correcting regionsAR1-ARk present the characteristics opposite to the characteristic ofaberration caused by the optical disc 9, i.e., such that the aberrationis corrected.

However, the electric fields (Eg1-Egk) produced in liquid crystalelement portions corresponding to the gaps P1-Pk, Q1-Qk between therespective transparent electrodes vary depending on the thickness of theinsulating layers 123, 124. FIG. 20 shows the intensity of the electricfield on the interfaces of the insulating layers and the liquid crystalelement portions, when the respective electrodes (here A1-A6) areapplied with voltages, with the thickness of the insulating layers (THi)used as a parameter. In the graphs, it is assumed that the liquidcrystal element has a thickness of 5 micrometers (μm), and the gapsP1-Pk, Q1-Qk have a width of 5 μm. The intensity of the electric fieldis shown for three thicknesses (THi) of the insulating films, i.e.,THi=1, 6, 15 μm. Also, the respective electrodes are applied withvoltages in increment of 2 volts, i.e., A1=10 (V), A2=12 (V), . . . ,A6=20 (V).

As shown in FIG. 20, the electric field abruptly drops at the liquidcrystal element portions corresponding to the gaps with a smallerthickness of the insulating films (THi=1 μm). Therefore, the alignmentof the liquid crystal cannot be sufficiently changed in these portions.In other words, it is difficult to correct the aberration since no phasedifference can be produced in light passing therethrough. On the otherhand, the reduction in the electric field is smaller when the thicknessof the insulating films THi=6 μm. The electric field exhibits asubstantially flat profile when the thickness of the insulating filmsTHi=15 μm, so that the aberration can be sufficiently corrected by theliquid crystal element even in the gaps.

FIG. 21 shows an example of wave front aberration (spherical aberration)produced by the optical disc 9. The amount of phase difference (innanometer: nm) in a light incident plane of the liquid crystal element114 is plotted in the radial direction with reference to the value atthe center of the liquid crystal element 114. More specifically, a lightbeam incident on the liquid crystal element 114 has the phase differenceof the profile shown in FIG. 21 with respect to the radial direction ofthe liquid crystal element 114. The aberration can be corrected byapplying predetermined voltages to the liquid crystal element 114 forcanceling out the phase difference to cause an in-plane phase change inthe incident light beam.

FIG. 22 shows a phase difference (nm) produced in a light beam passingthrough the liquid crystal element 114 by a predetermined voltageapplied to each electrode (A1-Ak, C1-Ck), when the insulating layer 123sandwiched by the liquid crystal element 114 and the electrode layer 112is smaller in thickness than the liquid crystal element 114 (forexample, the thickness of the insulating layer 123 is equal to or lessthan approximately 1 μm, while the thickness of the liquid crystalelement 114 is 5 μm). The phase difference is plotted with reference tothe values in the gaps. In the aberration correcting region (AR1-ARk) ofthe liquid crystal element 114 corresponding to each electrode, adesired phase difference is obtained by the application of the voltage.On the other hand, since the electric field intensity is small inregions of the liquid crystal element 114 corresponding to the gapsP1-Pk, Q1-Qk, a phase change hardly occurs. Thus, abrupt discontinuityoccurs in the phase difference, thereby significantly impeding thecorrection for the aberration.

FIG. 23 shows a phase difference produced in a light beam passingthrough the liquid crystal element 114 by the application ofpredetermined voltages when the insulating layer 123 of the liquidcrystal element 114 has a thickness equal to or larger than thethickness of the liquid crystal element 114. As shown in the graph, thephase difference in a region of the liquid crystal element 114corresponding to each of the gaps P1-Pk, Q1-Qk shows the value betweenthe phase differences produced by two adjacent electrodes, so that theaberration is sufficiently corrected. It should be noted that the amountof phase difference is optimally coincident with the phase differencecurve shown in FIG. 21 in the most favorable conditions.

FIG. 24 shows a phase difference produced in a light beam passingthrough the liquid crystal element 114 by the application ofpredetermined voltages, when the insulating layer 123 is smaller inthickness than the liquid crystal element 114 (for example, thethickness of the insulating layer 123 is approximately 3 μm, while thethickness of the liquid crystal element 114 is 5 μm). As shown in thegraph, the phase differences in the gap regions are larger than the caseshown in FIG. 22. More specifically, the aberration correcting unit ofthe present invention is improved to ensure sufficient phase differenceseven in the gap regions and enables the correction for aberration inthese regions, which has not been possible in the prior art. It shouldbe noted that the value of the phase difference required in each gapregion depends on a variety of conditions and parameters such as aparticular optical system, the type of disc, a control circuit, and soon for use therewith. Therefore, the thickness of the insulating layersmay be determined to ensure desired phase differences in accordance withthe conditions. In other words, the thickness of the insulating layersmay be determined such that an electric field applied to a portion ofthe liquid crystal element corresponding to each gap region is equal toor larger than a predetermined intensity.

It should be noted that at least one of the insulating layers sandwichedbetween the liquid crystal element 114 and the electrode layers may beformed sufficiently thick, for example, thicker than the liquid crystalelement, and both insulating layers sandwiching the liquid crystalelement need not be formed thick.

As described above, sufficient phase changes can be provided even in thegap regions, thereby making it possible to accurately correct theinfluence of aberration occurring in an optical path.

While the foregoing embodiments have been described from a viewpoint ofthe correction for aberration of reflected light caused by the opticaldisc, the aberration correcting optical unit according to the presentinvention may be disposed at any position as long as it is on theoptical path from the light source to the photodetector. Also, theaberration correcting optical unit according to the present invention isnot limited for spherical aberration and coma aberration, but may beapplied to a correction for a variety of aberrations such asastigmatism. Further, the present invention can be applied to an opticalpickup apparatus which has a plurality of light sources or a pluralityof optical paths. For example, the present invention can be applied toan optical pickup apparatus which includes light sources such as atwo-wavelength laser or the like having different wavelengths forrecording and reproducing CD and DVD, respectively.

According to the present invention, as described in detail, an electrodelayer for applying voltages to the optical element (liquid crystal) isconfigured such that an appropriate electric field should be applied toa portion of the optical element corresponding to a portion between theplurality of electrodes (i.e., a gap) in order to correct aberration. Inparticular, the electrode layer is configured to have a multi-layerstructure or an insulating layer of appropriate thickness is providedbetween the optical element and the electrode layer.

As is apparent from the foregoing, the present invention can providehigh performance aberration correcting optical unit and an opticalpickup apparatus capable of accurately correcting aberration occurringin an optical path.

The invention has been described with reference to the preferredembodiments thereof. It should be understood by those skilled in the artthat a variety of alterations and modifications may be made from theembodiments described above. It is therefore contemplated that theappended claims encompass all such alternations and modifications.

1-16. (canceled)
 17. An aberration correcting optical unit disposed inan optical path of an optical system for irradiating a recording mediumwith a light beam emitted from a light source and for guiding areflected light beam reflected by said recording medium, for correctingaberration occurring in said reflected light beam, comprising: a liquidcrystal element for causing a phase change to light passing therethroughby the application of voltage; first and second electrode layersopposing to each other, said first and second electrode layerssandwiching said liquid crystal element; said first electrode layerincludes a plurality of divided electrodes electrically isolated fromone another by gaps in the same plane, and an insulating layer disposedbetween said liquid crystal element and said first electrode layer, saidinsulating layer having a thickness larger than the gaps of said firstelectrode layer.
 18. An aberration correcting optical unit according toclaim 1, wherein said insulating layer has a thickness larger than athickness of said liquid crystal element.
 19. An aberration correctingoptical unit according to claim 1, further comprising a liquid crystalalignment layer provided between said liquid crystal element and saidinsulating layer.
 20. An aberration correcting optical unit according toclaim 1, further comprising: a second insulating layer disposed betweensaid liquid crystal element and said second electrode layer, whereinsaid second electrode layer includes a plurality of divided electrodeselectrically isolated from one another by gaps in the same plane, andsaid second insulating layer having a thickness larger than the gaps ofsaid second electrode layer.
 21. An aberration correcting optical unitaccording to claim 4, further comprising a liquid crystal alignmentlayer provided between said liquid crystal element and said secondinsulating layer.
 22. An optical pickup apparatus including theaberration correcting optical unit according to claim 1, comprising: alight source for emitting a light beam; an optical system forirradiating a recording medium with the light beam emitted from saidlight source and for guiding a reflected light beam reflected by saidrecording medium; and a photodetector for detecting said reflected lightbeam.